Langmuir turbulence (LT) plays an important role in enhancing vertical mixing in the ocean surface boundary layer. Such enhancement is strongly affected by the diurnally varying heat flux, especially in the early morning when there is a transition from cooling to heating. In this period, turbulence is weakened yet the surface heat flux is changing rapidly, such that the deviation of transient turbulence from its equilibrium state is large. This may lead to biases in the parameterization of turbulent mixing in large scale ocean circulation models in which an equilibrium of turbulence state with the surface forcing is often assumed. In this study, we focus on the transient response of Langmuir turbulence to sudden changes in surface forcing using large eddy simulations (LES), and compare it with the transient response of shear-driven and convective turbulence. After a sudden change in the surface forcing, the turbulence intensity decreases rapidly at first and then gradually reaches a new equilibrium state. This initial decrease seems to result from the formation of a shallower warm layer near the surface, which hinders the downward transport of turbulent kinetic energy. Being blocked from the surface driving forces, turbulence away from the surface dissipates rapidly. With shear and turbulence intensity increasing near the surface, the initial surface warm layer eventually mixes downward and the turbulence adjusts to a new equilibrium state. Turbulence structures during such transitioning period are analyzed and the timescale of the transient response is estimated . These analyses may help improving the vertical mixing parameterizations in the ocean surface boundary layer.

Langmuir turbulence, transient response, transient buoyancy forcing